Because Milankovitch cycles cannot explain climate variability over the time scale that we're interested in predicting climate. Milankovitch cycles can explain large variations in climate over very long time scales, scales of thousands of years. Milankovitch cycles do not explain variability in climate on the scales of hundreds or tens of years. Variability at smaller time scales is driven by other factors, such as carbon dioxide (CO2) and other greenhouse gas concentrations.

Variations in the Earth's orbit around the sun are called Milankovitch cycles. Scientist do not believe that Milankovitch cycles can start or end an ice age. Each cycle runs a different length and the variations in the cycles reinforce their effects or cancel them.

Axial progression takes approximately 26,000 years. This is one of the Milankovitch cycles.

Axial progression takes approximately 26,000 years. This is one of the Milankovitch cycles.

Axial progression takes approximately 26,000 years. This is one of the Milankovitch cycles.